1. Field of the Invention
This invention relates to a method and apparatus for remotely measuring features in geometrically distorted images, in particular digital photographic and thermal images of wind turbine blades. The invention facilitates such inspections from the ground, on operating wind turbine generators and has utility for remotely detecting propagating latent defects, existing damage and broken adhesive bonds within the skin of a wind turbine blade. This permits subsurface defects to be detected before they become too large for repair in situ, which provides significant economic advantages, as the cost of repairing the wind turbine blade in situ is typically 10% of the cost of replacing a blade.
2. Description of the Related Technology
Due to their large size, extensive surface area and complex shape, wind turbine blades are difficult to non-destructively inspect even within a fabrication or repair facility. Visual inspection cannot identify defects below the surface of the outer skin of the wind turbine blade, which typically is fabricated from a fiberglass material. Active thermography inspection techniques using heat are effective for near surface defects but can give false positives and false negatives due to variations in material thickness and surface emissivity.
Shearography with either thermal stress or during flexure testing of the blade in the factory can be used to detect fiberwaves in spar caps and other areas of the blade, but the technique is slow, expensive and is usually performed only if known issues are suspected. Angle beam ultrasonic techniques are very slow and may not work through thick carbon fiber spar caps.
As a result, blades are commonly installed on towers and put into service with a significant probability of latent manufacturing defects. Furthermore, composite blades are subject to extreme temperature variations. Entrapped water in blades can undergo freeze/thaw cycles, which can cause internal damage. Cyclic forces of gravity and varying forces from the wind acting on the blades as they rotate can cause fatigue damage or the propagation of latent defects over time while manufacturing process mistakes can lead to early blade failure. Defects can grow below the surface of a wind turbine blade to the point that by the time cracks and damage breach the surface and can be detected visually, the damage may not be repairable on tower.
Detecting progressive subsurface damage and propagating defects in wind turbine blades in situ is difficult for a number of different reasons. Inspectors using sky cranes or rope access are expensive, time consuming and put personnel in a very dangerous working environment. While on tower, close access allows inspectors to visually detect blade defects such as trailing edge splits, cracks, lightning damage and blade erosion. In addition, major subsurface delaminations, cracks and debonded adhesive joints can easily go undetected with current technology.
Access to a wind turbine blade in situ with portable instruments for nondestructive testing also requires rope access or sky platforms and cranes. Blade and tower crawlers with nondestructive testing sensors for in situ inspection have been developed and tested, but they can be prohibitively expensive, slow to operate, require repair and maintenance themselves. Their effectiveness is also questionable. Thermal imaging of blades using solar heating during the transition from day to night has been attempted but is very limited in both the time over-which data may be taken and by being limited to blades facing the sun. Further this technique requires stopping the rotors, with consequential loss of revenue.
Helicopter access is both expensive and dangerous in wind farms, and no means are included to quantitatively measure or locate indications. Thermal imaging of blades using solar heating during the transition from day to night is very limited by both the time over-which data may be taken and by being limited to blades facing the sun. Finally, it is common practice to use optical and digital photographic imaging of blades in an attempt to detect visible damage from the ground. Again these methods suffer from complex logistics, insensitivity to defects, poor repeatability and do not allow precise measurement of the defect size, area or the location.
There accordingly exists a need for a fast, cost effective nondestructive inspection system and method for wind turbine blades using digital imaging to detect latent and propagating damage, which includes a method and apparatus for precision measurement of features or anomalies and locating these on the target blade.